Conventional sharpenable pencil barrels have been fabricated from natural substances such as wood, wood flour, treated cellulosic fibers, and synthetic materials such as plastic polymers.
Manufacturing of wooden and wood-product pencils usually involves multi-step techniques. Such procedures have become expensive, due to the increasing cost of raw materials, excessive time consumption, and the number of rejects encountered.
Because of the prohibitive impact of rapidly increasing operational costs, many pencil manufacturers have replaced wood with synthetic materials to make the pencil barrels or sheaths.
In the cosmetic industry when employing synthetics for the fabrication of pencil sheaths or barrels, certain requirements are necessary in order to yield the desired finished products. For instance, the composition of the synthetic barrel material must be compatible with that of the particular cosmetic core to prevent contamination of the cosmetic product by the synthetic or vice versa. Also, the selected material must be sharpenable, of adequate rigidity and resistant to normal atmospheric conditions in order to maintain a desired barrel shape.
As indicated above, polymeric materials, particularly thermopolymers, have been used in place of wood to make the pencil barrels. Because of the nature of such plastics, manufacturers have developed numerous extrusion techniques for manufacturing pencil barrels.
Some extrusion methods for producing pencil barrels are disclosed in U.S. Pat. Nos. 2,790,202; 2,988,784; 3,875,088; 3,936,519; 3,993,408; and 4,176,978. The barrel compositions respectively include thermoplastic binders and plasticizers in combination with other ingredients such as wood flour, a waxy substance, and a lubricant. Other extrudable compositions are disclosed in U.S. Pat. Nos. 3,875,088 and 3,993,408, wherein thermoplastic binders are utilized in combination with a cellulosic fibrous filler and a water-soluble metallic soap.
Crystal, U.S. Pat. No. 3,936,519 discloses a technique of extruding a foamed plastic sheath around a graphite-clay lead which has applied thereto a heat-activatable adhesive. In Crystal, the inside diameter of the sheath is fixed by the diameter of the rod-like, solid graphite-clay lead co-extruded therewith.
The pencil sheath compositions and respective methods in all of the above-cited prior art, except those in U.S. Pat. Nos. 3,936,519 and 4,176,978, do not employ blowing or foaming agents. Furthermore, the compositions thereof are designed for, and limited to, use in conventional or crystalline graphite pencils.
U.S. Pat. No. 4,413,921 discloses the use of polyolefins such as polyethylene, polypropylene, and equivalent derivatives thereof to form sharpenable cosmetic pencil barrels.
While some of the thermoplastic-based cosmetic pencil barrels of the prior art have proven to be generally acceptable, the commercial success of others has been somewhat limited, due to inherent drawbacks. Problems such as undesirable breakage, due to brittleness often occur. Another major problem often encountered with some of the pencil barrels of the prior art is that of poor, non-uniform sharpenability. Poor sharpenability can result in the penetration of the cosmetic marking core by small barrel chips or shavings. With respect to the extruded cosmetic pencil barrels, certain problems still persist. Typical of such problems is that of "weld lines", i.e. the undesirable, nonaesthetic striations on the outer wall, caused by use of conventional "spider-type" extrusion pins. Additional problems often encountered in some of the extrusion methods of the prior art are those such as non-uniformity of the outer wall surface of the pencil barrels and the poor printability thereof. Difficulty in maintaining uniform wall thickness within narrow limits is also a frequently incurred problem. The problem of non-uniform barrel wall thickness combined with those of poor internal structural integrity often results in penetration of the solvents from the marking core through the walls of the pencil barrel. The foregoing problems are often directly associated with the particular extrudable composition and the extrusion process selected therefor, especially the die shaping and cooling steps.
The above-mentioned combined factors of composition and extrusion conditions are interdependent. Hence, in order to be useful in the production of cosmetic pencils, the barrels utilized therein must possess the desired characteristic of good compatibility with the cosmetic marking core material, have outstanding sharpenability, printability, good concentricity, uniform wall thickness, good impermeability, and storageability.